The present invention relates to probe measurement systems for measuring the electrical characteristics of integrated circuits or other microelectronic devices at high frequencies.
There are many types of probing assemblies that have been developed for the measurement of integrated circuits and other forms of microelectronic devices. One representative type of assembly uses a circuit card on which the upper side are formed elongate conductive traces that serve as signal and ground lines. A central opening is formed in the card, and a needle-like probe tip is attached to the end of each signal trace adjacent the opening so that a radially extending array of downwardly converging needle-like tips is presented by the assembly for selective connection with the closely spaced pads of the microelectronic device being tested. A probe assembly of this type is shown, for example, in Harmon U.S. Pat. No. 3,445,770. This type of probing assembly, however, is unsuitable for use at higher frequencies, including microwave frequencies in the gigahertz range, because at such frequencies the needle-like tips act as inductive elements and because there are no adjoining elements present to suitably counteract this inductance with a capacitive effect in a manner that would create a broadband characteristic of more or less resistive effect. Accordingly, a probing assembly of the type just described is unsuitable for use at microwave frequencies due to the high levels of signal reflection and substantial inductive losses that occur at the needle-like probe tips.
In order to obtain device measurements at somewhat higher frequencies than are possible with the basic probe card system described above, various related probing systems have been developed. Such systems are shown, for example, in Evans U.S. Pat. No. 3,849,728; Kikuchi Japanese Publication No. 1-209,380; Sang et al. U.S. Pat. No. 4,749,942; Lao et al. U.S. Pat. No. 4,593,243; and Shahriary U.S. Pat. No. 4,727,319. Yet another related system is shown in Kawanabe Japanese Publication No. 60-223,138 which describes a probe assembly having needle-like tips where the tips extend from a coaxial cable-like structure instead of a probe card. A common feature of each of these systems is that the length of the isolated portion of each needle-like probe tip is limited to the region immediately surrounding the device-under-test in order to minimize the region of discontinuity and the amount of inductive loss. However, this approach has resulted in only limited improvement in higher frequency performance due to various practical limitations in the construction of these types of probes. In Lao et al., for example, the length of each needle-like tip is minimized by using a wide conductive blade to span the distance between each tip and the supporting probe card, and these blades, in turn, are designed to be arranged relative to each other so as to form transmission line structures of stripline type. As a practical matter, however, it is difficult to join the thin vertical edge of each blade to the corresponding trace on the card while maintaining precisely the appropriate amount of face-to-face spacing between the blades and precisely the correct pitch between the ends of the needle-like probe tips.
One type of probing assembly that is capable of providing a controlled-impedance low-loss path between its input terminal and the probe tips is shown in Lockwood et al. U.S. Pat. No. 4,697,143. In Lockwood et al., a ground-signal-ground arrangement of strip-like conductive traces is formed on the underside of an alumina substrate so as to form a coplanar transmission line on the substrate. At one end, each associated pair of ground traces and the corresponding interposed signal trace are connected to the outer conductor and the center conductor, respectively, of a coaxial cable connector. At the other end of these traces, areas of wear-resistant conductive material are provided in order to reliably establish electrical connection with the respective pads of the device to be tested. Layers of ferrite-containing microwave absorbing material are mounted about the substrate to absorb spurious microwave energy over a major portion of the length of each ground-signal-ground trace pattern. In accordance with this type of construction, a controlled high-frequency impedance (e.g., 50 ohms) can be presented at the probe tips to the device under test, and broadband signals that are within the range, for example, of DC to 18 gigahertz can travel with little loss from one end of the probe assembly to another along the coplanar transmission line formed by each ground-signal-ground trace pattern. The probing assembly shown in Lockwood et al. fails to provide satisfactory electrical performance at higher microwave frequencies and there is a need in microwave probing technology for compliance to adjust for uneven probing pads.
To achieve improved spatial conformance between the tip conductors of a probe and an array of non-planar device pads or surfaces, several high-frequency probing assemblies have been developed. Such assemblies are described, for example, in Drake et al. U.S. Pat. No. 4,894,612; Coberly et al. U.S. Pat. No. 4,116,523; and Boll et al. U.S. Pat. No. 4,871,964. The Drake et al. probing assembly includes a substrate on the underside of which are formed a plurality of conductive traces which collectively form a coplanar transmission line. However, in one embodiment shown in Drake et al., the tip end of the substrate is notched so that each trace extends to the end of a separate tooth and the substrate is made of moderately flexible nonceramic material. The moderately flexible substrate permits, at least to a limited extent, independent flexure of each tooth relative to the other teeth so as to enable spatial conformance of the trace ends to slightly non-planar contact surfaces on a device-under-test. However, the Drake et al. probing assembly has insufficient performance at high frequencies.
With respect to the probing assembly shown in Boll et al., as cited above, the ground conductors comprise a pair of leaf-spring members the rear portions of which are received into diametrically opposite slots formed on the end of a miniature coaxial cable for electrical connection with the cylindrical outer conductor of that cable. The center conductor of the cable is extended beyond the end of the cable (i.e., as defined by the ends of the outer conductor and the inner dielectric) and is gradually tapered to form a pin-like member having a rounded point. In accordance with this construction, the pin-like extension of the center conductor is disposed in spaced apart generally centered position between the respective forward portions of the leaf-spring members and thereby forms, in combination with these leaf-spring members, a rough approximation to a ground-signal-ground coplanar transmission line structure. The advantage of this particular construction is that the pin-like extension of the cable's center conductor and the respective forward portions of the leaf-spring members are each movable independently of each other so that the ends of these respective members are able to establish spatially conforming contact with any non-planar contact areas on a device being tested. On the other hand, the transverse-spacing between the pin-like member and the respective leaf-spring members will vary depending on how forcefully the ends of these members are urged against the contact pads of the device-under-test. In other words, the transmission characteristic of this probing structure, which is dependent on the spacing between the respective tip members, will vary in an ill-defined manner during each probing cycle, especially at high microwave frequencies.
Burr et al., U.S. Pat. No. 5,565,788, disclose a microwave probe that includes a supporting section of a coaxial cable including an inner conductor coaxially surrounded by an outer conductor. A tip section of the microwave probe includes a central signal conductor and one or more ground conductors generally arranged normally in parallel relationship to each other along a common plane with the central signal conductor so as to form a controlled impedance structure. The signal conductor is electrically connected to the inner conductor and the ground conductors are electrically connected to the outer conductor, as shown in FIG. 1. A shield member is interconnected to the ground conductors and covers at least a portion of the signal conductor on the bottom side of the tip section. The shield member is tapered toward the tips with an opening for the tips of the conductive fingers. The signal conductor and the ground conductors each have an end portion extending beyond the shield member and the end portions are able to resiliently flex, despite the presence of the shielding member, relative to each other and away from their common plane so as to permit probing devices having non-planar surfaces.
In another embodiment, Burr et al. disclose a microwave probe that includes a supporting section of a coaxial cable including an inner conductor coaxially surrounded by an outer conductor, as shown in FIGS. 2A, 2B, and 2C. A tip section of the microwave probe includes a signal line extending along the top side of a dielectric substrate connecting a probe finger with the inner conductor. A metallic shield may be affixed to the underside of the dielectric substrate and is electrically coupled to the outer metallic conductor. Ground-connected fingers are placed adjacent the signal line conductors and are connected to the metallic shield by way of vias through the dielectric substrate. The signal conductor is electrically connected to the inner conductor and the ground plane is electrically connected to the outer conductor. The signal conductor and the ground conductor fingers (connected to the shield via vias) each have an end portion extending beyond the shield member and the end portions are able to resiliently flex, despite the presence of the shielding member, relative to each other and away from their common plane so as to permit probing devices having non-planar surfaces. While the structures disclosed by Burr et al. are intended to provide uniform results of a wide frequency range, they unfortunately tend to have non-uniform response characteristics at high microwave frequencies.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.